If you like creating sounds, Reaktor is what you're looking for. The possibilities of the program are as close to endless as makes no difference. It's supplied with a library of ready-made instruments and tools, which are all useful and fascinating in their own right, but the real Reaktor experience starts when you fire up a blank instrument and start to roll your own.
You can start off with something relatively simple, like a basic subtractive or FM synth, or dive straight into alien territory with a granular synthesis module. Rip open an existing instrument and cannibalize the parts. Bolt some new controllers onto your favourite synth. Take out the parts you don't need to save on CPU cycles. Head over to the user library and download hundreds of free user-created devices, from self-generating noise organisms to faithful, labour of love reproductions of vintage equipment. Forget about sound altogether and use the math and XY modules to create a Mandelbrot Set generator or a racing game - both have been done.
Essentially, if you get bored with Reaktor, there's not much hope. The downside (aside from the expense) is that it's all too easy to get lost in sound creation and parameter tweaking instead of actually making any music. But, really, you won't mind. Too much fun.
You can now buy Reaktor Session for a reduced price - it's a cut-down version that doesn't allow for the creation of new modules, but does give you access to the hundreds of library or user-created ones. There are a lot of bug rumours attached to Session, though - and Reaktor itself is not renowed as a highly stable application - which is probably unavoidable, given its scope.
As of Reaktor 4 (the retail version at time of writing) and Session, Reaktor runs on OS X, in addition to OS 9 and Windows.